1. Field of the Invention
The present invention relates to a flyback converter, and more particularly to a flyback converter with a leakage-inductance energy recycling circuit.
2. Related Art
FIG. 1 shows a conventional flyback converter 100, in which a DC input voltage Vin is coupled to a primary winding Lp of a transformer Tx, a power transistor Q1 and the primary winding Lp of the transformer Tx are connected in series, and the power transistor Q1 is an MOS. A controller 102 outputs a pulse width modulation signal to switch the ON and OFF of the power transistor Q1, so as to convert a DC input voltage Vi to the secondary winding Ls of the transformer Tx into an output voltage Vo through the ON or OFF of the power transistor Q1. The sensing resistor R2 and the primary winding Lp of the transformer Tx are connected in series to detect a current Ip of the primary winding.
Once the power transistor Q1 is switched from ON to OFF under the control of the pulse width modulation signal, a drain voltage Vd of the power transistor Q1 shown in FIG. 2 generates a transient high voltage at the instant when turning off the power transistor Q1, and meanwhile ripples are generated, which may result in the breakdown and damage of the power transistor Q1. In order to solve such a problem, a clamping circuit is generally disposed at the primary winding Lp of the transformer Tx.
A clamping circuit 104, electrically connected to the primary winding Lp of the transformer Tx, includes a first resistor R1 and a first capacitor C1 connected in parallel, a first end of the first resistor R1 and the first capacitor C1 is connected to a first end of the primary winding Lp of the transformer Tx, a cathode of the first diode D1 is connected to a second end of the first resistor R1 and the first capacitor C1, and an anode of the first diode D1 is connected to a second end of the primary winding Lp.
Once the power transistor Q1 is switched from On to Off under the control of the pulse width modulation signal, an energy ½LLKIP2 of leakage-inductance LLK stored in the primary winding Lp of the transformer Tx charges the parasitic capacitor Cds between a drain and a source of the power transistor Q1. After the drain voltage Vd is raised to the sum of a voltage Vc1 across two ends of the first capacitor C1 and the DC input voltage Vin, the first diode D1 is conducted, and at this time, the leakage-inductance current LLLK charges the first capacitor C1 through the first diode D1. In such a manner, a drain voltage Vd of the power transistor Q1 is clamped at (Vc1+Vin) by the clamping circuit, and thus preventing the power transistor Q1 from being broke-down or damaged. Next, the first diode D1 is switched to OFF due to the natural resonance, the leakage-inductance LLK and the stray capacitance of the circuit enters the resonance state, and the energy stored in the first capacitor C1 is discharged through the first resistor R1, and thus the energy is dissipated in the form of heat.
The power transistor Q1 is periodically turned on or off under the control of the pulse width modulation signal, so that the charge-discharge process is periodically repeated. As the energy stored in the first capacitor C1 is dissipated in the form of heat after the discharging process, there are excessive heat energy accumulated within the flyback converter, and the energy stored in the first capacitor C1 is also wasted.